The present invention relates to an optical receiver for receiving an optical signal and converting it into an electric signal and, in particular, to an optical receiver for receiving analog high-frequency optical signals used in CATV (cable television) and the like.
An optical receiver comprises a light-receiving device such as a photodiode (PD) which receives an optical signal and generates a corresponding current signal, and a preamplifier which converts this current signal into a voltage signal and amplifies the thus-converted signal to the amplitude required for a television receiver or the like connected downstream. As a results of increasing demand for a larger number of channels for CATV or the like, the frequency band which such an optical receiver can cover is extending into the higher frequency region and is presently approaching 1 GHz.
In order to treat such a high-frequency signal, impedance matching in any transmission system for transmitting the signal becomes quite important. In particular, when a signal is to be transmitted on a transmission line without impedance matching, the signal may be reflected at a point of impedance mismatch, thus causing noise or signal distortion, thereby degrading the reliability of transmission.
The above-mentioned impedance matching will now be studied in an optical receiver. The output impedance of a PD ranges from several hundred ohms to several thousand ohms in a frequency band of several tens of MHz to 1 GHz. On the other hand, it is difficult to design a preamplifier that is suitable for amplifying analog high-frequency signals with such a high input impedance. In general, an amplifier having an input impedance of 50 ohms or 75 ohms is employed, while the PD is terminated with a resistor of 50 ohms or 75 ohms, thus simulatively realizing impedance matching. This method, however, is unfavorable in terms of noise characteristics since the equivalent input noise of the amplifier is increased. Therefore, an impedance-matching transformer having a primary impedance of several hundred ohms and a secondary impedance of 50 ohms or 75 ohms is often inserted between the PD and the preamplifier. The preamplifier is connected to the secondary terminal of the matching transformer by way of an interstage coupling capacitor Cc.
Here, the matching transformer has a core made of a magnetic material having excellent high-frequency characteristics and is shaped like a torus, two windings are wound around the core, the number of the respective windings corresponding to the predetermined impedance values, and one end of one winding is connected to one end of the other winding. In general, the characteristics of a circuit in a frequency region as high as 1 GHz cannot be accurately determined unless parasitic capacitance of devices and circuit elements, their parasitic inductance, and the like are taken into consideration.
A PD is equivalently expressed by a current source Is supplying a current corresponding to an incident optical signal, junction capacitance Cj of a semiconductor connected in parallel thereto, and a diffused resistor Rj connected in series to a thus-formed parallel circuit. When a PD chip is assembled into a package, it is further necessary to take into account the parasitic inductance Ls of the bonding wire connecting the PD chip to the lead pin of the package, and the parasitic capacitance Cs formed between the lead pin and the outer lid of the package or the like. By so doing, the combined equivalent circuit of the PD and the package can be expressed in a lumped constant fashion. When a thin line having a diameter of several tens of micrometers is employed as a bonding wire, the wire has an inductance component of about 1 nH/mm. At a frequency of 1 GHz, this inductance component acts as an impedance of several ohms to several tens of ohms, which is on the same order of magnitude as that of the diffused resistor Rj and thus cannot be neglected.
Assuming that the optical-to-electrical conversion efficiency of the PD and the gain of the preamplifier are free from frequency dependence, the optical signal transfer characteristic Vo/Is for such an equivalent circuit of an optical receiver in the lower frequency region is determined by the input impedance of the preamplifier and the interstage coupling capacitor Cc connected between the preamplifier and the transformer. In the higher frequency region, on the other hand, parasitic elements of the PD, the inductance of the matching transformer, the parasitic capacitance between the windings, the loss in the transformer core, and the like have a complex relationship with the transfer characteristics. As a consequence, the gain in the transfer characteristic begins to decay gradually at about 100 MHz, such that, at 1 GHz, it is as much as 7 to 8 dB weaker than in the smaller frequency region.
In order to compensate for the above-mentioned decrease in the higher frequency region, the conventional method as disclosed, for example, in European Patent Application Laid-Open No. 372,742, has been to insert a frequency characteristic correcting coil Lc between the PD and the matching transformer. This Lc causes blunt resonance to occur with respect to the circuit element Cj, Cs, or the parasitic capacitance of the matching transformer. It has consequently been effective in increasing the gain in the band of several hundred MHz. However, in order for this method to increase the gain in a frequency region as high as about 1 GHz so as to flatten its band characteristic, it has been necessary to set the junction capacitance Cj of the PD, its parasitic capacitance Cs, the parasitic capacitance of the matching transformer, and the like to very small values.
It has consequently been necessary to employ a high-cost packaging method in which, for example, the PD chip is mounted on a ceramic chip carrier, or in which an air core coil is employed as the frequency characteristic compensating coil Lc, so as to reduce the parasitic capacitance. But when using a PD package which can be aligned easily with or fixed to an optical fiber, or when using ordinary components such as transformers or coils that can be surface-mounted, it has only been possible to achieve a band width of 600 MHz or lower, thus failing to match a band width of 700 MHz or higher used in optical CATV.
Therefore, it is an object of the present invention to improve high-frequency characteristics of optical receivers by use of circuit technique without the aid of solutions such as the use of special packages and improvement, in high-frequency characteristics, of the above light-receiving devices themselves.
The optical receiver in accordance with the present invention comprises a light-receiving device, having one terminal coupled to a bias power supply, for converting an optical input signal into an electric signal; a preamplifier, coupled to the other terminal of the light-receiving device, for amplifying the electric signal; and an impedance-matching transformer, electrically coupled to the light-receiving device and the preamplifier, having a primary impedance matching the output impedance of the light-receiving device and a secondary impedance matching the input impedance of the preamplifier. In order to overcome the above-mentioned problems, the light receiving device further comprises a first frequency characteristic compensating circuit is coupled between the bias power supply and one terminal of the light-receiving device. The first circuit that compensates frequency characteristics comprises an inductance component to compensate the frequency characteristic. When the above-mentioned frequency characteristic compensating circuit is thus coupled between the PD bias power supply terminal and the bias power supply, the gain near the upper limit of the band can be enhanced without losing its middle to low frequency characteristics.